An image reading device is used to read image information recorded in a document. Such an image reading device is provided in a variety of image handling apparatuses, such as, for example, a document scanner, facsimiles, digital copiers, and devices that combining several functions of the foregoing.
An image reading device typically includes a reading unit to optically read image information from a document by irradiating light onto the document, which may be placed on a document table, or may be delivered by an automatic document feeder, for example. To that end, a reading unit generally includes a light source and an image sensor that receives the light reflected from the document. Some image sensors may also convert the received reflected light into an electric signal.
Occasionally, during reading of a document by the reading unit, the image detected by the image sensor may be an inaccurate representation of the original image of the document. For example, the so-called “shading phenomenon” refers to the situation where an original image of a document that otherwise exhibiting uniform density may nevertheless result in non-uniform image sensor output. This phenomenon may be a product of many possible causes, including, for example, the non-uniformity of light emitted from the light source, the tolerance in precision of optical elements or in the assembly thereof, the optical or electrical characteristics of the image sensor, and the like.
Conventional attempts for addressing the above may include the provision of a criterion member, or a calibration image, of known characteristics, e.g., image density, which is located within the movement range of the reading unit, and which is read by the image reading device prior to reading a document in order to obtain criterion or calibration shading data. The criterion shading data is then used to compensate the image data acquired from a document.